Oh, this is similar to what I meant actually! Sometimes key phrases come into my head that I feel 'yes, this perfectly conveys what this character is trying to say at the moment', so I write it down, but I also have to make notes about the surrounding context (which aren't perfectly phrased, but felt necessary to me because sometimes the timing/beats of the scene is what makes those 'key phrases' sound perfect to by brain). And while it's not a full script from beginning to end, it adds up and before I know it I've been writing for 2 hours 
I also used to think 'if it's really that good surely it'll stick with me long enough for me to write it down tomorrow', but I've woken up with the feeling that 'I came up with something good last night but now I can't remember it for the life of me' too many times now to trust that thought :'D Even if I jot down the important bits/'key phrases', it'll just be a bunch of out-of-context phrases the next day with most of the magic gone XD